In the current flip-chip technology, electrode pads are formed on a surface of a surface mount device (SMD) such as a semiconductor integrated circuit (IC) chip, and corresponding electrical connection pads are provided on an organic package substrate. Solder bumps or other electrically conductive adhesive materials are formed between the chip and the package substrate so as to allow an active surface of the chip to be mounted on the package substrate in a face-down manner, such that the solder bumps or electrically conductive adhesive materials serve as input/output (IO) connections for electrically and mechanically connecting the chip to the package substrate.
Besides the chip, other surface mount devices such as a passive component can also be mounted on and electrically connected to the same package substrate so as to improve electrical performances. However, for accommodating the different surface mount devices, electrical connection pads on the package substrate need to be coated with solder of different heights and sizes, such that the electrical connections with different heights are formed to be correspondingly electrically connected to various types of the surface mount devices.
Further, when a packaging process for the package substrate, the chip and the passive component is subsequently performed, a plurality of solder balls are usually implanted on a surface of the substrate to electrically connect the substrate to an external electronic device. A solder material is pre-formed on electrical connection pads of the substrate where the solder balls are to be implanted so as to effectively mount the solder balls on the substrate.
A stencil printing technique is commonly employed to form such solder material on the substrate. Referring to FIG. 1 to conduct the stencil printing technique, primarily a package substrate 10 is provided, wherein on a surface of the package substrate 10 there are formed a solder mask layer 11 and a plurality of electrical connection pads 12 for defining locations of the solder material such as solder paste (not shown). A stencil 13 having a plurality of grids 13a is mounted on the package substrate 10. After the solder material is placed on the stencil 13, a roller 14 is used to repeatedly roll on the stencil 13; or a spraying process is performed to spray the solder material over the stencil 13. As a result, the solder material can be deposited in the grids 13a to form solder (not shown) on the electrical connection pads 12 after the stencil 13 is removed. Subsequently, under a reflow-temperature condition in which the solder would melt, the solder is subjected to a reflow-soldering process to form soldering elements (not shown) on the electrical connection pads 12 of the package substrate 10, and the soldering elements can be electrically connected to an external electronic device.
However, in order to coat solder of different heights and sizes on the electrical connection pads of the package substrate to form the electrical connections with different heights for being electrically connected to various types of the surface mount devices, the foregoing stencil printing process should be performed 2 to 3 or even more times, and also the reflow-soldering process needs to be carried out 2 to 3 or even more times. This makes the fabrication processes rather complicated and time-consuming, thereby significantly increasing the fabrication cost.
Moreover, since each of the electrical connection pads is partially covered by the solder mask layer located between the pads, the exposed part of each of the electrical connection pads is reduced in size. This not only causes a problem of subsequently aligning the solder bumps to the electrical connection pads but also makes the solder difficult to be attached to the electrical connection pads, such that the yield of the stencil printing technique would be degraded and an overflow of the melted solder material may occur during the reflow-soldering process.
Furthermore, to precisely apply the solder material using the stencil printing technique, besides the accurate size of the stencil, the times and cleanness of the stencil printing process being performed should also be concerned. Due to viscosity of the solder material, the more times of the stencil printing process being performed, the more residues of the solder material on walls of the grids of the stencil are. This would cause the amount and shape of the solder material in the next time of stencil printing to be different from those required for the electrical connection pads. Therefore, practically after the stencil printing process has been performed a certain number of times, the stencil must be wiped and cleaned, otherwise it would lead to the problem of the inaccurate shape and size of the solder material, and cause an inconvenience in fabrication, as well as degrade the quality and reliability of products.
Accordingly, the problem to be solved here is to provide a method for fabricating electrical connections with different heights on a circuit board by easily and effectively coating solder of different heights on electrical connection pads of the circuit board, so as to allow the electrical connections with different heights on the circuit board to be electrically connected to different types of electronic elements.